1. Field of the Invention
The present invention relates to a solid-state image sensor, and more particularly to a solid-state image sensor with reduced smear and noise.
2. Description of the Related Art
The solid-state image sensor has a silicon substrate of a first conductivity type. The substrate has a photoelectric converter region of a second conductivity type and a drain region of the second conductivity type. The photoelectric converter region and the drain region are separated by a channel region, which underlies a transfer gate. If charges are injected from the substrate into the drain region, then smear and/or noise may appear on an image sensed by the sensor. In order to have suppressed smear and noise, it was proposed that the photoelectric convener region is deeper in junction depth than the drain region.
Japanese laid-open patent publication No. 61-99472 discloses such a proposal as described above. FIG. 1 is a schematic cross sectional elevation view of a conventional solid-state image sensor The solid-state image sensor has a p-type silicon substrate 105. An n-type photo-diode region 101 is selectively provided in an upper region of the p-type silicon substrate 105. A drain region 104 is also selectively provided in another upper region of the p-type silicon substrate 105, wherein the drain region 104 is separated from the n-type photo-diode region 101 by a channel region. A transfer gate electrode 103 is provided which overlies the channel region between the drain region 104 and the n-type photo-diode region 101. A p-type low impurity concentration region 102 is selectively provided, which underlies the n-type photo-diode region 101, so that an entire bottom face of the n-type photo-diode region 101 is bounded by the p-type low impurity concentration region 102 from the p-type silicon substrate 105.
Smear charges, which may cause smear, may be generated in the p-type silicon substrate 105. The generated smear charges are drawn into the p-type low impurity concentration region 102, and the smear charges are then captured into the n-type photo-diode region 101, whereby it is prevented that the smear charges are drawn into the drain region 104, resulting in that smear is suppressed. The smear charges generated in the silicon substrate 105 as balk region include electrons and holes, for example, injected electrons and holes into the silicon substrate as well as electrons and holes generated from pixels of the solid-state image sensor.
The above conventional structure prevents that smear charges generated in the silicon substrate 105 are drawn to the drain region 104, but does not prevent that smear charges generated in the silicon substrate 105 are drawn to the n-type photo-diode region 101.
In the above circumstances, it had been required to develop a novel solid-state image sensor free from the above problem.
Accordingly, it is an object of the present invention to provide a novel solid-state image sensor free from the above problems.
It is a further object of the present invention to provide a novel solid-state image sensor, which shows smear-reduced images.
It is a still further object of the present invention to provide a novel solid-state image sensor, which shows noise-reduced images.
It is yet a further object of the present invention to provide a novel solid-state image sensor, which prevents smear charges generated in a silicon substrate from being drawn to not only a drain region but also a photoelectric converter region.
The present invention provides a solid-state image sensor comprising: a semiconductor bulk region of a first conductivity type; at least a drain region of a second conductivity type selectively provided in the semiconductor bulk region; at least a photoelectric converter region of the second conductivity type selectively provided in the semiconductor bulk region, and the photoelectric converter region being separated from the drain region; and at least a potential barrier region of the first conductivity type selectively provided in the semiconductor bulk region, the potential barrier region being adjacent to at least a part of bottom and side faces of the photoelectric converter region, and the potential barrier region having a higher impurity concentration than the semiconductor bulk region.
The potential barrier region prevents charges generated in the semiconductor bulk region from being drawn into the photoelectric converter region. The potential barrier region also prevents leakage of charges from the photoelectric converter region to the semiconductor bulk region. The potential barrier region reduces smear and noise of images.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.